Polyolefin compositions having elastic properties while maintaining a good thermoplastic behavior have been used in many application fields, due to the valued properties which are typical of polyolefins (such as chemical inertia, mechanical properties and nontoxicity). Moreover, they can be advantageously transformed into finished products with the same techniques used for thermoplastic polymers.
In particular, flexible polymer materials are widely used in the medical field (e.g. in the production of containers for plasma or intravenous solutions, or phleboclysis tubes), as well as for packaging, extrusion coating and electrical wires and cables covering.
In many of these applications, vinyl chloride polymers containing adequate plasticizers, which are necessary to give said polymers their desired flexibility characteristics, are presently used. Said polymer products, however, are subject to ever increasing criticism both for the suspected toxicity of the plasticizers they contain and because when incinerated, they can disperse into the atmosphere extremely toxic by-products, such as dioxin. It would be very useful, therefore, to substitute said materials with products which besides the desired flexibility characteristics and, optionally, transparency, would have the chemical inertia and nontoxicity typical of olefin polymers.
Elastic polypropylene compositions retaining good thermoplastic behavior has been obtained in the art by way of sequential copolymerization of propylene, optionally containing minor quantities of olefin comonomers, and then ethylene/propylene or ethylene/alpha-olefin mixtures. Catalysts based on halogenated titanium compounds supported on magnesium chloride were commonly used for this purpose.
For instance, EP-A-400 333 describes elastoplastic polyolefin compositions obtained by sequential polymerization comprising:    A) 10-60 parts by weight of a crystalline polymer or copolymer of propylene;    B) 10-40 parts by weight of a polymer fraction containing ethylene, insoluble in xylene at ambient temperature; and    C) 30-60 parts by weight of an ethylene/propylene copolymer fraction, soluble in xylene at room temperature.
Said compositions are relatively flexible and have good elastic properties, as demonstrated by flexural modulus lower than 700 MPa values (the compositions prepared in the working examples of this document show flexural modulus values ranging from 270 to 350 MPa), associated to good tension set values; nevertheless, such values are not fully satisfactory for many applications and the compositions do not have satisfactory optical properties, such as transparency. Satisfactory transparency characteristics can be obtained in compositions prepared by sequential polymerization, when the content of the crystalline polymer produced in the first stage of polymerization is relatively high; nevertheless, the presence of high amounts of crystalline fraction is detrimental to elasticity and flexibility properties.
EP-A-373 660 describes polypropylene compositions, obtained by sequential polymerization, containing (A) from 70 to 98% by weight of a crystalline copolymer of propylene and (B) from 2 to 30% by weight of an elastomeric copolymer of ethylene with propylene and/or another alpha-olefin, whose portion soluble in xylene has an intrinsic viscosity which satisfies a particular correlation with the one of the crystalline copolymer. These compositions however, although having good optical qualities, show high flexural modulus values (typically higher than 600 MPa), due to the high content of crystalline copolymer (A). WO 00/11057 describes a process for the preparation of heterophasic polyolefin compositions comprising:    i) a polymerization stage carried out in the presence of a Ziegler-Natta catalyst, to produce a) a crystalline homo or copolymer of propylene and/or ethylene, and b) an elastomeric copolymer of ethylene with a C3-C10 alpha-olefin;    ii) a treatment stage to deactivate the Ziegler-Natta catalyst and to add a metallocene catalyst; and    iii) a polymerization stage carried out in the presence of the metallocene catalyst, to produce an elastomeric copolymer of ethylene with a C3-C10 alpha-olefin, preferably containing 50-75% by weight of ethylene.
EP-A-472 946 describes flexible elastoplastic polyolefin compositions comprising, in parts by weight:    A) 10-50 parts of an isotactic propylene homopolymer or copolymer;    B) 5-20 parts of an ethylene copolymer, insoluble in xylene at room temperature; and    C) 40-80 parts of an ethylene/propylene copolymer containing less than 40% by weight of ethylene and being soluble in xylene at room temperature; the intrinsic viscosity of said copolymer is preferably from 1.7 to 3 dl/g. Said compositions have a flexural modulus of less than 150 MPa; a tension set, at 75% elongation, of 20-50%, and at 100% elongation of about 33-40%; Shore D hardness from 20 and 35, and Shore A hardness of about 90. These mechanical properties, though advantageous with respect to the compositions known in the prior art, are still not fully satisfactory for certain applications. In particular, more flexible polymers are required for medical applications, as well as for packaging, for calendered materials, for extrusion coating, and for electrical wires and cables covering.
Therefore, there is a need for more flexible polyolefin compositions, having lower flexural modulus values, while maintaining acceptable optical properties.